1. Field of the Invention
This invention relates to magnetite particles and a process for producing the same. More particularly, it relates to magnetite particles each of which has silicon components in the interior and on its surface respectively, and is improved in properties, i.e. electrical resistance, remaining magnetization and fluidity, in a well-balanced way by controlling the amount (in terms of silicon) of the silicon component exposed on the surface, and is suited for use principally as powdery material for magnetic toner for electrostatic copying and as black pigment powder for coating materials. It also relates to a process for producing the particles.
2. Prior Art
Magnetite particles obtained by aqueous solution reaction have recently been widely used as magnetic toner materials for dry electronic copiers, printers, etc. The magnetic toner is required to possess various general developing characteristics. The advance of electrophotography in recent years has encouraged rapid progress in copying machines and printers, especially those using the digital technology, with the consequence that the requirements for performances of the magnetic toner have become more severe, The apparatus is required to output not merely characters as in the past but also graphics, photographs, etc. Some latest printers offer a print quality of 400 or more dots per inch, producing finer and clearer latent images on the photoreceptor. To keep pace with this, accordingly high fine-line reproducibility in development has been strongly desired.
Second, the magnetite particles themselves are required to have a high enough electrical resistance to stabilize the image density during electrostatic transfer of an image.
Magnetite particles which meet these requirements have been proposed. As improvements pertaining to the first problem, U.S. Pat. No. 4,992,191 and Japanese Patent Application Laid-Open Gazettes Nos. (sho.)61-155223 (155223/1986) and (sho.)62-278131(278131/1987) disclose magnetite particles containing a silicon component in their interior only. These particles are not fully satisfactory as yet in providing image quality with improved fine-line reproducibillty. Moreover, the disadvantage of low electrical resistance common to conventional magnetite particles remains unremedied.
Further, the magnetite particles disclosed by the publications also have other disadvantages that are poor fluidity and a tendency of the packed powder to excessively increase in density by vibrations during transportation and other handling, thereby seriously affecting the workability in the subsequent process of toner preparation.
As an improvement regarding the second problem, Japanese Patent Application Laid-Open Gazette No. (sho.)54-139544(139544/1979) teaches increasing the electrical resistance of magnetite particles by coating the surface with a silicon component. However, the approach again fails to improve the electrical resistance satisfactorily. It cannot meet the recent requirement for finer line reproduction of images, especially due to the inability to improve the remaining magnetization.